1. Field of the Invention
The present invention relates generally to a power amplifier. More particularly, the present invention relates to a power amplifying apparatus based on envelope elimination and restoration (EER) using push-pull switching.
2. Description of the Related Art
A power amplifier (PA) is generally used to amplify a transmit signal in a wireless communication system. In terms of cost, the PA is given a great deal of weight. Many developers of wireless communication systems are making efforts to develop a PA with high efficiency. Yet, it is not easy to develop a PA satisfying all of high efficiency, broad band, and high power requirements. To raise the efficiency of the PA, envelope tracking (ET) and envelope elimination and restoration (EER) have been suggested. However, it is hard to enhance both the efficiency and the band characteristic because of the high-level implementation.
The PA based on the EER approach is constructed as shown in FIG. 1. An envelope detector 110 detects and outputs an envelope of an input radio frequency (RF) signal, and a phase detector 120 detects and outputs a phase of the input RF signal. Namely, the RF signal is amplified separately in the amplitude and the phase. The drain bias of a switching mode PA 140, which amplifies the phase, is the output of an envelope amplifier 130. Hence, when the switching mode PA 140 operates in the saturation mode, the envelope of the output of the switching mode PA 140 conforms to the output of the envelope amplifier 130.
As shown in FIG. 2, the envelope amplifier 130 includes a voltage amplifier 210, a resistor 220, an operational (OP) amplifier 230, and a switching amplifier 240. The voltage amplifier 210 has a class-B push-pull structure in general and behaves as a voltage source. The class-B push-pull amplifier generally exhibits low efficiency but can be designed to achieve a wide bandwidth. The output of the OP amplifier 230 is determined by the direction of current flowing through the resistor 220. A control signal of the switching amplifier 240 is a pulse width modulation (PWM) signal. The switching amplifier 240 may employ a buck converter, which behaves as a current source. When the control signal of the switching amplifier 240 is positive, a switch 242 in the switching amplifier 240 is in the on-state, and the current flowing through the inductor 244 increases. Conversely, when the control signal of the switching amplifier 240 is negative, the switch 242 is in the off-state, and the current flowing through the inductor 244 decreases.
Typically, a buck converter has a relatively high efficiency of over 80%. However, as the switching frequency gets higher, loss also increases, thereby rendering the buck converter as being unsuitable for high frequency operations. Therefore, the envelope amplifier 130 includes the switching amplifier 240 for processing the low frequency band of the signal and the voltage amplifier 210 for processing the high frequency band of the signal. When detecting the envelope of the RF signal, since the bandwidth increases by more than two times the baseband signal, the overall performance of the envelope amplifier 130 is greatly influenced by the amplification efficiency of the low frequency band. Since the efficiency of the switching amplifier 240 is relatively high, when the switching amplifier 240 is able to process signals in the band as wide as possible, the total performance of the PA is enhanced. However, as the switching frequency of the signal fed to the switching amplifier 240 gets higher, the loss increases, thereby degrading the total efficiency instead.